The present invention generally relates to quality control of electrically thin semiconductor films (herein also referred to either as semiconductor thin films or thin films), and more particularly, to a method and apparatus for characterizing the quality of electrically thin semiconductor films and their interfaces. The phrase "electrically thin semiconductor film" is defined herein to be any semiconductor film for which there exists some surface potential such that a change in this potential results in a change in the potential at the semiconductor/substrate interface. In the semiconductor manufacturing industry, an effective way to characterize electrically thick semiconductor films is to make an electrical device on the material and then perform measurements using the device. The phrase "electrically thick semiconductor film" is defined herein to be any semiconductor film for which the potential at the semiconductor/substrate interface is independent of the surface potential for all values of the surface potential. This electrically thick film characterization technique is more sensitive than any other material diagnostic technique. The electrical device typically made on the semiconductor material for this purpose is a capacitor. Capacitance versus voltage, or C-V, measurements are made using the capacitor to characterize the quality of the electrically thick semiconductor films.
In the past, the capacitor has been made in an electrically thick semiconductor film and the C-V measurement was made by providing electrical contacts on opposite sides of the insulating substrate supporting the semiconductor layer, thus using the insulating substrate as the gate material One electrical contact was provided by the semiconductor layer itself available on the topside of the substrate. The other electrical contact was provided on the opposite side of the insulating substrate, i e. on the backside.
For the case where the insulating substrate is very thick (such as quartz or sapphire) the steps performed to provide such backside electrical contact were, first thinning the substrate (to a thickness of approximately 200 to 400 microns) and, then, evaporating metal on the backside of the thinned substrate to form the contact. The thinning step had to reduce the substrate to a thickness which would give interpretable capacities. Several problems are presented with this manner of making electrical contacts with a capacitor on the semiconductor material when taking the required C-V measurements. One problem is that the substrate thinning process is time consuming. Another problem is that the thinning process itself may affect the interface between the insulating substrate and semiconductor layer so as to change its electrical properties and thereby produce distorted results. A further problem is that even after thinning, thousands of volts of bias are required to accomplish the measurements. An additional problem is that the signal-to-noise ratio of the data is small because the thinned substrate is still many times thicker than the semiconductor film. Finally, as a consequence of the small signal-to-noise ratio this measurement has been restricted to doped, electrically thick semiconductor films.
For the case where the substrate is thin enough initially such that thinning is not required (such as Self-Implanted-OXide or SIMOX) the backside electrical contact is made either directly to a conducting substrate if one is used such as, for example silicon in the case of SIMOX, that is supporting the insulator or to an evaporated conductor such as aluminum. The problem with this type of technique is that it has been restricted to electrically thick semiconductor films. To make such films usually requires special processing thus increasing the complexity and cost of the measurement.
Consequently, there has been a long-felt need to devise a more reliable, quicker and simpler measurement technique for characterizing the quality of electrically thin semiconductor films.